This invention relates to automatic gain control circuits used in conjunction with gain control amplifiers and in particular to automatic gain control circuits having a variable and selectable range of coverage.
It is conventional practice to provide gain control amplifiers in radio receivers in order to effectively receive signals having diverse strengths as well as provide stability and minimize distortion. Maximum amplification may be provided for very weak signals, minimum amplification may be provided for strong signals, and controlled amplification may be provided for signals of intermediate strengths.
The usual automatic gain control circuit requires a filter or integrating capacitor to prevent undesired variation of automatic gain control voltage with modulation. A delay in the restoration of sensitivity of the receiver when the level of the incoming signal is suddenly decreased is caused by a charge retained on the capacitor. In U.S. Pat. No. 3,230,458 a circuit was disclosed that included a differentiator for detecting a sudden decrease of signal level and short circuit means responsive to the decrease exceeding a predetermined rate for momentarily short circuiting the filter capacitor. The controlled receiver is instantly restored to full sensitivity and the capacitor becomes charged quickly as required to control the signal for obtaining the desired output level.
The prior art automatic gain control circuits were limited in ranges of their time constants and the time constants were not easily changed or reprogrammed.